A Mini fleet is typically 5 to 15 vehicle though it can be as few as 2 vehicles. A true fleet is 15+ vehicles. They are especially good for when you have a larger number of vehicles that need insuring. It's a good way of managing a larger number of vehicles and keeping things simple. It is often seen as a way of saving money to insure multiple vehicles, sometimes this is true, sometimes this is not the case, especially on smaller fleets.

Typical advantages of fleets include:

Flexibility:

1. Vehicle which may be expensive or hard to insure in isolation can usually be accommodated. 
2. You can typically add a whole range of vehicle from cars, to vans, to trucks to unusual vehicle types. 
3. Fleets often allow a much wider range of drivers such as any driver (over a certain age). This may be hard to obtain on a single vehicle. 

Simplicity

Fleets are also super easy to maintain due to their common renewal date allowing vehicles to be simply added or removed without needing to issue a new policy. 

If you have all your vehicles insured as single vehicle and one has a claim, you then need to record the claim on all other policies which may lead to a premium increase. Eventually this can cause your premiums to skyrockets. 

With a fleet, you declare claims etc as the start and then it's reassessed at renewal. They are more forgiving on a smaller number of claims than individual policies as mentioned above.

Fleet Experience vs No Claims Bonus

Fleets differ to a single vehicle policy in that they use a fleet experience. This records the profitability of the fleet as a whole and shows an insurer how good (or bad) a risk you are. A single vehicle will typically earn a no claims bonus which increases or decreases according to claim free driving. As a general rule, the less a fleet costs an insurer, the less the annual premium will be - If you are costing an insurer an average of £3000 per vehicle for example, expect to pay more than this per vehicle on the fleet. For this reason, insurers typically ask for at least 3 years fleet experience (though more is better) and will put a price per vehicle according to how the fleet has performed. A large claim may be more forgiving on a no claims bonus based policy where a fleet policy may be more forgiving on several smaller claims than a no claims bonus based policy.

It should be borne in mind that fleets are not so good for smaller number of vehicles. Whilst a mini fleet may be convenient and offer many benefits, typically for under 5 vehicles the greatest benefirts are often not experienced.